It got me thinking that there are probably a lot of powerboaters who have run for years in coastal, tidal areas without worrying too much about how much fuel they’re burning. Sailors, on the other hand, especially those under sail, have always had to fret about the effects of current, mainly because they travel well under 10 knots most of the time, and are exposed to the effects of current for longer on a given trip.

With 600 horses pushing you through the water, why worry about the effect of a foul current? Because through-the-water is not the same as over-the-bottom.

So Peter’s article, which passes along a nice arithmetic lesson in fuel savings from Eldridge’s, will come in handy if you’re a powerboater who is (justifiably) worried about rising fuel costs and how to run more efficiently. Just to amplify a bit on the ideas in that lesson, here is another, more basic primer on how current can be friend or foe.

Let’s say you want to run 30 nautical miles to a favorite destination. If the water is slack, and you run at 15 knots, the trip will take you exactly two hours. Your speed through the water will be the same as your speed over the bottom. Now figure on a one-knot current directly against you. You run at the same rpm, and your speed through the water is the same – 15 knots. But as your GPS knows, and faithfully reports, your speed over the bottom is 14 knots. In two hours you’ll “lose” two miles; the trip will take about 8 minutes longer, and you will have burned that much more fuel.

If you can catch the fair current instead, you’ll be going 16 knots over the bottom at the same rpm that pushes you at 15 knots in slack water, and you’ll get to your destination 8 minutes sooner. A nice advantage, and the longer the run, the nicer it is.